1. Field of the Invention
The present invention relates to medical devices for monitoring vital signs, e.g., arterial blood pressure.
2. Description of the Related Art
Conventional vital sign monitors are used throughout the hospital, and are particularly commonplace in high-acuity areas such as the intensive care unit (ICU), emergency department (ED), or operating room (OR). Patients in these areas are generally sick and require a high degree of medical attention, typically provided by a relatively high ratio of clinicians compared to lower-acuity areas of the hospital. Outside the ICU and OR, clinicians typically measure vital signs such as systolic, diastolic, and mean arterial blood pressures (SYS, DIA, MAP), respiratory rate (RR), oxygen saturation (SpO2), heart rate (HR), and temperature (TEMP) with portable or wall-mounted vital sign monitors. It can be difficult to effectively monitor patients in this way, however, because measurements are typically made every few hours, and the patients are often ambulatory and not constrained to a single hospital room. This poses a problem for conventional vital sign monitors, which are typically heavy and unwieldy, as they are not intended for the ambulatory population. To make a measurement, a patient is typically tethered to the monitor with a series of tubes and wires. Some companies have developed ambulatory vital sign monitors with limited capabilities (e.g. cuff-based blood pressure using oscillometry and SpO2 monitoring), but typically these devices only make intermittent, rather than continuous, measurements. And even these measurements tend to work best on stationary patients, as they are easily corrupted by motion-related artifacts.
Most vital signs monitors feature a user interface that shows numerical values and waveforms associated with the vital signs, alarm parameters, and a ‘service menu’ that can be used to calibrate and maintain the monitor. Some monitors have internal wireless cards that communicate with a hospital network, typically using protocols such as 802.11b/g.
One of the most important parameters measured with vital signs monitors is blood pressure. In critical care environments like the ICU and OR, blood pressure can be continuously monitored with an arterial catheter inserted in the patient's radial or femoral artery. Alternatively, blood pressure can be measured intermittently with a cuff using oscillometry, or manually by a clinician using auscultation. Most vital sign monitors perform both catheter and cuff-based measurements of blood pressure. Blood pressure can also be monitored continuously with a technique called pulse transit time (PTT), defined as the transit time for a pressure pulse launched by a heartbeat in a patient's arterial system. PTT has been shown in a number of studies to correlate to SYS, DIA, and MAP. In these studies, PTT is typically measured with a conventional vital signs monitor that includes separate modules to determine both an electrocardiogram (ECG) and SpO2. During a PTT measurement, multiple electrodes typically attach to a patient's chest to determine a time-dependent ECG component characterized by a sharp spike called the ‘QRS complex’. The QRS complex indicates an initial depolarization of ventricles within the heart and, informally, marks the beginning of the heartbeat and a pressure pulse that follows.
SpO2 is typically measured with a bandage or clothespin-shaped sensor that clips to a patient's finger and includes optical systems operating in both the red and infrared spectral regions. A photodetector measures radiation emitted from the optical systems that transmits through the patient's finger. Other body sites, e.g., the ear, forehead, and nose, can also be used in place of the finger. During a measurement, a microprocessor analyses both red and infrared radiation detected by the photodetector to determine the patient's blood oxygen saturation level and a time-dependent waveform called a photoplethysmograph (PPG). Time-dependent features of the PPG indicate both pulse rate and a volumetric absorbance change in an underlying artery caused by the propagating pressure pulse.
Typical PTT measurements determine the time separating a maximum point on the QRS complex (indicating the peak of ventricular depolarization) and a foot of the PPG waveform (indicating the beginning the pressure pulse). PTT depends primarily on arterial compliance, the propagation distance of the pressure pulse (which is closely approximated by the patient's arm length), and blood pressure. To account for patient-dependent properties, such as arterial compliance, PTT-based measurements of blood pressure are typically ‘calibrated’ using a conventional blood pressure cuff and oscillometry. Typically during the calibration process the blood pressure cuff is applied to the patient, used to make one or more blood pressure measurements, and then left for future measurements. Going forward, the calibration measurements are used, along with a change in PTT, to measure the patient's continuous blood pressure (cNIBP). PTT typically relates inversely to blood pressure, i.e., a decrease in PTT indicates an increase in blood pressure.
A number of issued U.S. patents describe the relationship between PTT and blood pressure. For example, U.S. Pat. Nos. 5,316,008; 5,857,975; 5,865,755; and 5,649,543 each describe an apparatus that includes conventional sensors that measure both ECG and PPG waveforms which are then processed to determine PTT.